Abstract

We report observations of the reactive molecular ions OH+, H2O+, and H3O+ towards Orion KL with Herschel/HIFI. All three N = 1−0 finestructure
transitions of OH+ at 909, 971, and 1033 GHz and both fine-structure components of the doublet ortho-H2O+ 111−000 transition at
1115 and 1139 GHz were detected; an upper limit was obtained for H3O+. OH+ and H2O+ are observed purely in absorption, showing a narrow
component at the source velocity of 9 km s−1, and a broad blueshifted absorption similar to that reported recently for HF and para-H18
2 O, and
attributed to the low velocity outflow of Orion KL.We estimate column densities of OH+ and H2O+ for the 9 kms−1 component of 9±3×1012 cm−2
and 7±2×1012 cm−2, and those in the outflow of 1.9±0.7×1013 cm−2 and 1.0±0.3×1013 cm−2. Upper limits of 2.4×1012 cm−2 and 8.7×1012 cm−2
were derived for the column densities of ortho and para-H3O+ from transitions near 985 and 1657 GHz. The column densities of the three ions
are up to an order of magnitude lower than those obtained from recent observations ofW31C and W49N. The comparatively low column densities
may be explained by a higher gas density despite the assumption of a very high ionization rate.

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The original publication is available at DOI: 10.1051/0004-6361/201015117